1. Field of the Invention
The present invention relates to an optical system including a diffraction optical element which is used at a plurality of wavelengths.
2. Related Art Statement
The diffraction grating (diffraction lens) with condensing function is well known to have a property which is not present in a conventional refraction lens, as shown hereinafter.
(1) A spherical waves can easily be generated, so that an aberration can effectively be compensated.
(2) The diffraction optical element can have no substantial thickness, so that it has a high freedom of design, and thus a compact optical system can be realized.
(3) Since the amount corresponding to Abbe""s number in refraction lens a becomes negative amount in the diffraction lens, chromatic aberration can be corrected effectively, by combining the diffraction lens with the refraction lens.
When the characteristic of such a diffraction lens is utilized, the property of the optical system can be improved. This is described in, for example, Binary Optical Technology; The Theory and Design of Multilevel Diffractive Optical Element, Gray J. Swanson, Technical Report 854, MIT Lincoln Laboratory, August 1989.
As described above, on the one hand, the diffraction optical element has a number of useful characteristics which are not present in the conventional refraction element, but on the other hand the refraction efficiency thereof is dependent on the wavelength, so that if the diffraction optical element is used as a lens element, a plurality of diffraction lights (plural focuses) are present, which is unfavorable. Then, in the diffraction lens, generally as shown in FIG. 8, a base material member 10 being transparent to the utilized wavelength, is provided with a blazed relief pattern 20 having a cross-sectional form of a saw tooth shape, thereby concentrating energy to the diffraction light of a particular order.
However, as shown in FIG. 8, when the cross-sectional form is made to have a saw-tooth shape, the wavelength for maximum energy concentration is different according to its groove depth, so that it can not be rendered to concentrate the energy of wave-band light having a certain wavelength width to the diffraction light of a special order. Such a phenomenon does not cause any problem, for example, in case of using a mono-chromatic light source, such as a laser, but in the optical system utilizing white light, such as a camera, if the diffraction efficiency is made optimized with the light of special wavelength, a problem that the diffraction efficiency with the other wavelengths becomes decreased is caused.
FIG. 9 shows a wavelength dependency of a diffraction efficiency of first order diffraction light, in which in the diffraction optical element has a cross-sectional form shown in FIG. 8, BK7 is used as a base material member 10, and a relief pattern 20 is formed with a groove depth in such a manner that the diffraction efficiency of the first order diffraction light becomes 100% with wavelength xcex=510 nm. It is found from FIG. 9 that in a range of xcex=400 nm to xcex=700 nm which is generally considered to be a visible wavelength range, the diffraction efficiency is decreased according as it is away from the optimized wavelength xcex=510 nm, and the decrease of the diffraction efficiency, particularly, becomes remarkable in a shorter wavelength region.
Such a problem, moreover, is not confined to a problem in which only the spectral transmittance becomes decreased. That is, in the wavelengths in which the groove is not optimized, the diffraction efficiency of required order light does not reach 100%, but diffraction light of an unrequired order is generated.
FIG. 10 shows unrequired order light of zero order light and second order light displaced in front of or behind the first order light, as produced by the blazed diffraction optical element having the wavelength depending property of the diffraction efficiency of first order diffraction light shown in FIG. 9, and FIG. 11 shows the wavelength depending property of diffraction efficiency of unrequired order light. As is found from FIG. 11, when the diffraction efficiency of first order diffraction light becomes decreased, the diffraction efficiency of second order diffraction light becomes increased at the shorter wavelength side rather than the optimized wavelength side, and the diffraction efficiency of zero order diffraction light becomes increased at the longer wavelength side rather than the optimized wavelength side. Particularly, the increase of second diffraction light becomes remarkable at the shorter wavelength range.
In this way, if unrequired order light is generated, in the optical system using light of a certain wavelength width, for example, in an imaging optical system using white light, a flare or a ghost is caused, thereby decreasing the property of optical system.
It is an object of the present invention to eliminate the above described disadvantages of the conventional optical system including a diffraction optical element.
It is another object of the present invention to provide an optical system including a diffraction optical element, in which in case of using a plurality of wavelengths or light of some wave-band, the decreasing of the property of the optical system accompanying a wavelength dependency of diffraction efficiency may be prevented and the generation of a flare or a ghost due to light of an unrequired order can be prevented effectively.
According to the present invention, there is provided an optical system for forming an image of an object by light having a given wavelength width, comprising at least one diffraction optical element and a diffraction light selection element for transmitting a diffraction light of a given order to an output side of the diffraction optical element and for attenuating the diffraction light of orders other than the given order.
In an embodiment of the image display system according to the present invention, the diffraction light selection element is constructed by a numerical aperture limiting member for limiting light flux and having a larger numerical aperture than the numerical aperture of a given magnitude. In this way, it is preferable to screen the light incident from outside a given solid angle, thereby decreasing the unrequired order light, selectively.
In a preferable embodiment of the optical system according to the present invention, it further comprises a refraction optical element, and the diffraction optical element is constructed by a blazed diffraction lens, and the diffraction optical element and the refraction optical element have the refraction power of the same sign. In this way, the diffraction light of the second order can be screened effectively.
As seen from FIG. 10, the problem due to an unrequired order of light is caused by a difference of diffraction angle between the required diffraction light and the unrequired order light. Specifically speaking, if a plurality of diffraction lights are generated in the diffraction lens included in the imaging optical system, generally, a plurality of images corresponding to respective diffraction lights are generated, thereby causing a problem of generating a spot flare or a ghost. In order to resolve such a problem, therefore, it is advantageous to shield the unrequired order light efficiently, by paying attention to the difference of diffraction angle between the required diffraction light and the unrequired order light.
By taking the above points into consideration, in the present invention, a diffraction light selection element for transmitting a diffraction light of given order and for attenuating the diffraction light of orders other than the given order, for example, a numerical aperture limiting member for limiting light flux having a larger numerical aperture than the numerical aperture of a given magnitude, is provided on the emanating side of the diffraction optical element.